The applicant has identified that current design high flow solenoids have an equal area spool, meaning that lands of the spool have the same external dimension, usually the outside diameter, resulting in the lands having the same surface area for driving the spool in response to fluid pressure against the lands. To increase the pressure obtained from the current design solenoid spool valves requires the spool diameter to be increased. As the diameters of each of the lands on the spool increase, the force against a diaphragm of the solenoid spool valve increases, necessitating a magnet (coil) size of an electromagnetic actuator to be increased. The applicant has determined that it would be desirable to obviate the necessity to increase the magnet (coil) size with pressure capacity of the solenoid spool valve.
Furthermore, the applicant has also identified that increasing diameters of all lands on the spool in accordance with current practice typically increases leakage to an exhaust port of the solenoid spool valve, requiring a larger pump to compensate for the leakage.
Examples of the invention seek to solve, or at least ameliorate, one or more disadvantages of previous solenoid spool valves.